Polyelectrolytes (PEs) possibly occur in the form of a pearl-necklace structure in poor solvents. This structure, characterized by the form of segregated beads connected by narrow strings, has been extensively studied by scaling analysis and computer simulation, and was believed to be formed by the Rayleigh instability mechanism. However, the structure has not been experimentally well studied, and a complete experimental picture on the necklace formation is still not available. In this study, pearl-necklace structures of DNA induced by different condensing agents are observed by atomic force microscopy. The results show that the formation of a pearl-necklace conformation is highly dependent on kinetic factors such as incubation time and thermodynamic factors including the concentration and condensing ability of the agent. If the incubation time is sufficiently short, this conformation can exist as an intermediate state of the DNA coil-globule transition. When agents with weaker crowding effect or screening ability are added, the structure appeared to be thermodynamically stable. The same effect was observed in rather low concentrations of agent. A rods-on-a-string structure, unlike the common characterized spherical beads-on-a-string conformation, was observed and was believed to result from the stiffness of DNA. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 971-979